Silicon-on-insulator semiconductor dice are used in applications where it is desired to provide a very effective insulating barrier between semiconductor devices formed in the die. A silicon-on-insulator (SOI) die consists of three layers: (1) an active layer, where the semiconductor devices are formed, (2) an insulating layer, and (3) a handle layer. The insulating layer is sandwiched between the active and handle layers. The active and handle layers are formed of silicon; the insulating layer is normally formed of silicon dioxide (frequently referred to simply as “oxide”). The insulating layer may also comprise of a sandwich of thermally grown oxides and deposited glasses or oxides. The handle layer is normally the thickest of the three layers. Since the active and handle layers are formed of silicon and therefore have the same temperature coefficient of expansion, the die is very stable thermally. Furthermore, since the insulating layer provides a strong insulating barrier, devices formed in the active layer can be electrically insulated from each other very effectively using isolation structures that extend through the active layer.
FIG. 1A shows a cross-sectional view of a conventional SOT-like semiconductor package 1 containing a SOI die 3. SOT, or “small outline transistor” is an industry standard package used in the packaging of discrete transistors or low pin count integrated circuits. SOI die 3 includes an active layer 3A, an insulating layer 3I and a handle layer 3H. SOI die 3 is mounted via the exposed surface of the handle layer 3H to a lead 2B. Devices (not shown) in active layer 3A are electrically connected to leads 2A and 2B via contact pads (not shown) at the surface of active layer 3A and via bonding wires 4A and 4B. “Gull wing” leads 2A and 2B protrude from molding compound 5. The lower surfaces of leads 2A and 2B are mounted to a printed circuit board (PCB) 6.
FIG. 1B shows a cross-sectional view of a conventional Double_No-Lead (DFN) semiconductor package 11 containing an SOI die 13. SOI die 13 includes an active layer 13A, an insulating layer 13I and a handle layer 13H. SOI die 13 is mounted via the exposed surface of the handle layer 13H to a heat slug 12B. Devices (not shown) in active layer 13A are electrically connected to leads 12A and 12C via contact pads (not shown) at the surface of active layer 13A and via bonding wires 14A and 14B. Leads 12A and 12C have external surfaces that are coplanar with the side and bottom surfaces of molding compound 15, i.e., unlike leads 2A and 2B in FIG. 1A, leads 12A and 12C do not protrude from the molding compound 15. The lower surfaces of leads 12A and 12C and heat slug 12B are used to mount package 11 to a PCB 16.
Unfortunately, the good electrical insulation properties of an SOI die create thermal problems, in that the heat generated by devices formed in the active layer is trapped by the insulating layer. In packages 1 and 11, for example, the active layers of SOI dice 3 and 13 are physically and thermally separated from the leads by the insulating and handle layers (the bonding wires are typically very thin and do not transfer significant amounts of heat). Materials such as silicon dioxide that are used for the insulating layer are often good thermal insulators. As a result, temperatures in the active layer may reach unacceptably high levels and may damage or destroy devices formed in the SOI die.
Thus it would be very useful to provide a semiconductor package for a SOI die that facilitates the removal of heat generated by semiconductor devices formed in the die. Such a solution would allow SOI dice to live up to their full potential from both an electrical and thermal standpoint.